Conventionally, radar devices, which detect target objects in all directions around a ship concerned, acquire signals (i.e., received signals) in a polar coordinate system while rotating a radar antenna at a predetermined speed (cycle). The radar device converts the received signal in the polar coordinate system into the pixel data in the rectangular coordinate system to write them in an image memory, and reads out each pixel data stored in the image memory at a predetermined timing. For this, an SDRAM is typically used for the image memory. A display has display dot groups arranged two-dimensionally, and it displays the pixel data while carrying out raster scans at a predetermined frequency (for example, 60 Hz).
FIGS. 7A and 7B are views showing a relation between pixel addresses DM of the conventional general image memory, and dot addresses DD of the display, where FIG. 7A shows the pixel addresses DM of the image memory, and FIG. 7B shows the dot addresses DM of the display.
As shown in FIGS. 7A and 7B, the conventional radar device is set such that a Col direction (column direction) of the image memory as an SDRAM and a raster-scan direction of the dot matrix of the display are in agreement. This is because the SDRAM data can read out data at a higher speed in the Col direction than in the row direction, and thereby it can keep up with the speed of the raster scan of the display.
Meanwhile, in the recent radar devices, plural modes in which their display styles are different from each other can be set for the ship concerned or target objects other than the ship, etc. For example, the following display modes can be selected: a display mode in which north is set as a reference direction and the reference direction is set to be upward in the display image (hereinafter, referred to as a “North-Up Mode”); a display mode in which the bow direction of the ship is set to be upward in the display image (hereinafter, referred to as a “Head-Up Mode”); and a display mode in which a predetermined azimuth, such as an azimuth to a destination or a bow azimuth at the time of setting the display mode, is set to be upward (hereinafter, referred to as a “Course-Up Mode”). In addition, the following modes can be selected: a mode in which a position of the ship is fixed at the center in the display image (hereinafter, referred to as a “Relative Motion”); and a mode in which the ship position is moved in the display image corresponding to the movement of the ship (hereinafter, referred to as a “True Motion”).
However, it may be necessary to change the display image corresponding to the combination of these modes or the turning of the ship. For this reason, disadvantages may arise when an angular difference is produced between the Col direction of the image memory and the raster-scan direction of the display, and the angular difference changes due to the turning of the ship.
FIGS. 8A and 8B are views for illustrating the disadvantages of the conventional display modes, where FIG. 8A is a view showing a relation between the Col direction of the image memory and the raster-scan direction of the display when these directions have a predetermined angle therebetween, and FIG. 8B is an enlarged view of the image memory for illustrating a method of reading out from the image memory. In FIG. 8B, black dots schematically show dots of the display, respectively.
As shown in FIGS. 8A and 8B, when the Col direction of the image memory differs from the scanning direction of the display, if data is intended to be read out along the raster-scan direction of the display, the pixel data must be read out across two or more lines (rows). Here, for the SDRAM, although the data can typically be read out at a high speed in the Col direction, they cannot be read out at a high speed in the Row direction. For this reason, it will be difficult to keep up the read-out speed of the pixel data with the transfer rate of the image required for image rendering by the raster scan.
Therefore, in the conventional radar device, it is necessary to coincide the Col direction and the raster-scan direction, and until the antenna makes one revolution to write new pixel data in the image memory, the pixel data written in the last time are read out as pixel data at the same position in the display as they are to display them. For this reason, when displaying a radar image in the Head-Up Mode, even if the bow direction of the ship is changed, until the antenna would make one revolution and all the pixel data of the image memory would be updated, the correct radar image corresponding to the bow direction of the ship cannot be displayed and, thus, the display image cannot smoothly follow the turning of the ship.
As a method of solving the disadvantage, JP3696502(B) discloses the following radar device. As shown in FIG. 2 of JP3696502(B), this radar device sets division blocks each including a predetermined number of pixels arranged two-dimensionally in the image memory, reads out pixel data for each block according to the timing of display, and then stores the data in the SRAM. Then, according to an angular difference in the Col direction of the image memory and the scanning direction of the display, the radar device reads out from the SRAM the necessary pixel data among the pixel data temporary stored in the SRAM to the display them.
However, in the method disclosed in JP3696502(B), the SRAM has to be provided in addition to the SDRAM which is normally used as the image memory. Therefore, the constituent components for the device will increase in number and its cost will thus increase. In addition, when reading out, the following two procedures must be performed: (1) a batch transmission from the SDRAM to the SRAM, and (2) a read-out from the SRAM corresponding to the display modes. Further, when reading out from the SDRAM of (1), pixel data of two or more rows of the image memory must be read simultaneously. For example, two or more SDRAMs must be arranged in parallel for each row, and processing such as reading out simultaneously from two or more SDRAMs must be performed. Therefore, it will increase the cost significantly.